1. Field of the Invention
The present invention relates generally to processing sheet material and, more particularly, to a sheet folding apparatus using fold rollers that are biased toward one another.
2. Background Information
Several systems for folding material are known in the art where the characteristics of particular folding components are adjustable. For instance, some systems allow for the manual adjusting of distances between folding rollers, as described in U.S. Pat. No. 5,190,514 (Galvanauskas), U.S. Pat. No. 5,242,364 (Lehmann), and U.S. Pat. No. 5,937,757 (Jackson et al.), the disclosures of which are hereby incorporated by reference in their entirety. In these systems, an operator must have knowledge of a material's properties (such as weight or thickness) before carefully adjusting the system to accommodate those properties.
Other folding systems include self-adjusting components, such as the system described in U.S. Pat. No. 5,738,620 (Ebner et al.), the disclosure of which is hereby incorporated in its entirety. In the Ebner patent, a stack of sheets is pushed between a pair of pre-folding rollers and a pair of folding rollers by a folding knife. One half of each roller pair is spring-loaded towards the other half and pivots away from the other half when a stack of sheets is introduced by the folding knife. While such a system allows for some automatic adjustment, much force is needed to force a stack of sheets between the rollers. Also, due to the orientation of the Ebner system, a stack of sheets can not be folded more than one time.
A system for finishing printed sheets into booklets is described in PCT Document No. WO 00/18583 (Trovinger et al.). The Trovinger PCT includes an operation where individual booklet sheets are folded using two drive motor assemblies. A first vertical drive motor assembly operates to immobilize a sheet by pressing it against a fold blade with a folder assembly. This first vertical drive motor assembly moves a set of fold rollers into contact with both the sheet and a longitudinal fold blade. The axes of rotation for the fold rollers are perpendicular to the fold blade used to fold each sheet. A second horizontal drive motor then operates to deform the sheet against the fold blade by reciprocating the set of fold rollers, which have been placed into contact with the sheet, back and forth along the fold blade to in effect crease the sheet. The number and spacing of these rollers are such that during horizontal movement of the fold rollers, at least one fold roller passes over every point along the portion of a sheet where a fold is to be formed.
The Trovinger PCT also describes the use of self-adjusting, v-shaped fold rollers, each of which include two complementary disks that are spring-loaded on a common axle. However, rollers of this shape and configuration may only be useful for folding a limited range of materials.
It would be desirable to provide for precise folding of a wide range of sheet materials where fold rollers are self-adjustable.